I had set a content type to allow it to be used as a simplenews newsletter, and then updated the JSON Feeds Processor so that it filled in the simplenews category for all the items.
However after realizing this wasn't what I needed, I removed the field simplenews from the content type and removed the option "use as newsletter" too, but all the newsletters still show up in the admin page.
I want to delete all of these old newsletters but there doesn't seem to be any option in the module to do that ?